An MRI examination is currently contraindicated for patients having an implanted cardiac pacemaker or defibrillator. This is due to the following types of problems that might arise during an MRI examination:                Heating near the electrodes connecting the generator to the patient's heart;        The forces and torques exerted on the device immersed in the high static magnetic field of the MRI machine; and        Unpredictable behavior of the device itself, due to exposure to the extreme magnetic fields.        
The present invention is directed to a solution to the first type of problem, namely, the heating problem that appears mainly close to the electrodes, located at the distal end of the leads that are connected to the generator.
Leads placed in an MRI imager act like antennas and collect the radio frequency field (RF) emitted by the imager. The frequency of this RF field is equal to the Larmor frequency of protons, that is f=42.56×B01 wherein B01, in Tesla, is the characteristic static induction of the imager. For typical static induction B0 of 1.5 T or 3 T, the RF frequencies generated by the imager are correspondingly about 64 MHz and 128 MHz, respectively.
Induced currents circulate in the conductors of the leads immersed in the RF field, therefore generating heat which in turn heats the surrounding blood and tissue. In fact, because heating at the electrodes is proportional to the density of current flowing in the latter, the smaller the surface of the electrode, the larger the current density and therefore the higher the heating of surrounding tissues.
In practice, depending on the configuration of the generator, the leads and the MRI imager, the temperature rise experimentally observed typically reaches from 8° C. (for carbon electrodes) to 12° C. (for metal electrodes), and as high as 30° C.
However, the temperature increase should not exceed what is specified in the EN 45502-1 standard and its derivatives, and preferably be less than 2° C. Indeed, a temperature rise of 4° C. causes a local cell death that has for immediate effect, among others, to irreversibly change the detection and stimulation thresholds.
It is certainly possible, as described in U.S. Patent Publication Nos. 2003/0204217 A1 and 2007/0255332 A1, to provide an MRI safety mode in which a protection circuit, provided at the generator connector isolates the circuit conductors of the generator, and connects the conductors to the ground of the generator to prevent circulation of parasitic induced currents. However, this approach is problematic because it prevents the device from remaining functional for the duration of the MRI examination. Because an MRI examination may last several minutes, it is therefore highly desirable that the device can continue seamlessly to detect depolarization potentials and deliver stimulation pulses to the myocardium as needed. This is so even though, during the MRI examination, the device may be switched to a particular secured mode of operation, notably disabling the circuitry sensitive to strong magnetic fields such as the RF telemetry circuits and the switching power supplies.
It is thus not considered to be sufficient in practice to simply disconnect and/or put ground all conductors of the lead during the MRI examination.
Another set of techniques previously proposed to reduce the currents induced by an MRI field are based on placing in series with the lead conductor(s) connected to the electrode(s), i.e., in the path of the induced currents, an impedance opposing the passage of the current during an MRI examination situation. It may be a single series coil (as disclosed in U.S. Pat. No. 7,123,013 B2). However, the mitigation, if it is significant, is not generally sufficient. It was also proposed to insert in the current loop a resonant circuit such as a tank circuit tuned to the RF frequency generated by the MRI imager, as described for example in Patent Publication Nos. 2009/0204171 A1 and 2007/0288058 A1. But this solution has the disadvantage of requiring as many filters as there are different characteristic frequencies of the imager types (e.g., 64 MHz, 128 MHz).
The EP 2198917 A1 and its counterpart US Patent Publication No. 2010/0160989 (both assigned to Sorin CRM S.A.S, previously known as ELA Medical) propose another approach, in the case of a bipolar lead, of disconnecting one of the conductors and connecting it to the ground of the generator housing, so that the grounded conductor can provide shielding protection for the other conductor, which remains functional. This technique is however restricted to bipolar leads; in addition, it requires a modification of the hardware design of the generator, so that the latter can achieve the necessary ground switching on detection of a MRI field.
Other known techniques propose to disable the electrode by a switch disposed in the vicinity thereof and controlled on detection by the generator of an MRI field detected by a sensor incorporated in the latter, as described in US Patent Publication No. 2010/0106227 A1. But the transmission of the command from the generator to the switch located in the lead head, at the other end, requires the presence of an additional conductor, hence the need to design a specific lead and also a specific generator and a specific connector. There is therefore no compatibility with existing leads or generators.
The US Patent Publication No. 2008/0154348 A1 proposes a passive, protection circuit located at the distal end of the lead and not requiring an additional connecting conductor. The principle is to place a PIN diode in parallel with a resistor in series with the electrode. The major drawback of this type of protection is that to compensate for the voltage drop across the diode (about 0.6 to 0.7 V) it is necessary to apply stimulation pulses having a far higher voltage than otherwise necessary. This need is permanent because the protection circuit is purely passive and must remain connected all the time. The loss in the protection diode can be more than half of the stimulation energy, with a major impact on the autonomy and useful lifetime of the device, whose duration may be reduced from 30 to 50%.
The U.S. Patent Publication No. 2009/0149909 A1 describes a comparable system for protection, wherein the lead conductor is provided at its distal end with a series resistor whose terminals are coupled to a “normally closed” switch, controlled by a controller analyzing the voltage at the terminals of this resistor. This device, like the one described above, not only has the disadvantage of creating losses that may affect the autonomy of the device, but also to be sensitive to spurious signals of high amplitude; in fact, any signal whose amplitude exceeds the threshold voltage of the controlled switch causes closure thereof and therefore neutralization of the series resistance of protection, with a corresponding risk of damage to heart tissue.